Influence of Masson pine canopy on characteristics of raindrops in red soil region of southern China.

Rainfall is the main factor influencing soil erosion, and rainfall kinetic energy is the most important source of power to soil erosion. After passing through forest canopy, characteristics of raindrops (raindrop size, terminal velocity and kinetic energy) change due to the redistribution of canopy and as a result, the characteristics of soil erosion on forest floor change. To explore the effect of canopy of Masson pine (Pinus massoniana), a typical tree species in red soil region of southern China, on soil erosion on forest floor, we observed five representative natural rainfall events at Changting County in Fujian Province, southern China, from May to June in 2014. Raindrop size was simultaneously measured by using filter paper method inside and outside the Masson pine forest, and raindrops inside and outside forest under different rainfall conditions were analyzed. The results showed that: 1) Raindrops inside forest were larger in number and wider in distribution range than those outside forest. The number of raindrops inside forest was 1 608, 19.5% more than that outside forest (1 346). Raindrop size distribution outside forest ranged from 0.6 to 5.7 mm, while that inside forest varied from 0.4 to 6.1 mm. 2) There was a clear difference in drop size distribution between inside and outside forest. Raindrop size inside forest conformed to a bimodal distribution, with the first peak around 1.5 mm in diameter and the second peak 3.6 mm. In contrast, raindrop size outside forest followed a unimodal distribution, with the peak around 1.8 mm in diameter. The median diameter of raindrops inside forest was 3.2 mm, 60.0% higher than that outside forest, which was 2.0 mm. 3) Terminal velocity of raindrops inside forest was similar to that outside forest when they were smaller than 2 mm in size; however, it was smaller inside than outside forest when drops were larger than 2 mm in size and the difference gradually increased from 0.6% to 2.0%. 4) Masson pine plantation was an applicable approach for vegetation restoration in red soil region of southern China. Rainfall kinetic energy was reduced by 21.8% after passing through canopy, mainly due to the canopy interception and canopy height. The results of our study could provide a reasonable basis for revealing soil erosion mechanism under Masson pine plantation in red soil region of southern China.